Fix uninitialize variable access

The patch 4d81260 has a trivial bug
that is uninitialized variable access just before set up the variable.

src/libuiomux/uiomux.c

@@ -569,12 +569,12 @@ uiomux_get_mem(struct uiomux *uiomux, uiomux_resource_t blockmask,
 	if ((i = uiomux_get_block_index(uiomux, blockmask)) == -1)
 		return 0;
 
+	block = &uiomux->blocks[i];
+
 	/* Invalid if no uio associated with it */
 	if (block->uio == NULL)
 		return 0;
 
-	block = &uiomux->blocks[i];
-
 	if (address)
 		*address = block->uio->mem.address;
 	if (size)